You can escape the brackets with a backslash so they are treated as regular characters. On that base, you could use the following regex to match all square brackets containing only digits:
\[[:digit:]*\]
When leaving the Replace with
box empty, a search/replace run should remove all footnote marks in your example.
Since only the opening bracket is a special character for LO Calc, the following should work, too:
\[[:digit:]*]